My cat gets mad when being pet in a certain spot on her side?

My cat is a spayed female tuxedo shorthair who is 8 years old (I've had her since she was 6 months old).  My initial concern started when my boyfriend went to pet my cat Ellie who was laying on her cat tower.  She got upset and tried to attack him (this is extremely unlike her, as she has never been aggressive towards anyone ever before).  When it happened, I took her to the vet to rule out any health concerns.  The vet performed a general physical exam and then did a full blood panel and also checked her urine.  The results all came back normal, so the cause behind this event wasn't really resolved.  About a week later I went to pet my cat while she was laying sideways and noticed that even though she was purring and seemed content, the moment my hand passed over a specific spot on her side, her pupils dilated and she looked angrily at my hand, possibly thinking about swatting at my hand.  She seems content for me to pet her anywhere except this one spot, and it seems to be on both of her sides.  The spot is basically around where her hips are, or where the back leg meets her side.  I'm starting to worry that even petting her lightly is inducing some sort of physical pain or discomfort.  If anyone has any ideas as to what could be causing this, please feel free to give me your thoughts.  

To add a little extra background, she is indoor only and no other pets are in the household.  Her eating and bathroom habits have not changed.  The only thing the vet noticed was slight weight gain over the last year (she is now 11lbs).

I suppose it's possible she is getting arthritis in her hips, as this sounds like the general area you are talking about.  I've never heard of people saying their cats responded this way to petting, but guess it's certainly possible.  You might try giving her some Cosequin, which can be sprinkled on her food.  You can get it at places like Petco or Petsmart, or Amazon.  Even if she doesn't have arthritis, that Cosequin won't hurt her.  Or if she seems to be in a LOT of pain, your Vet could prescribe pain meds.  You  might speak to them about this and see if they think it's possible that's what it is.

Thank you for the response, I'll call and talk to the vet about it and if arthritis is a possibility. She doesn't seem to be in pain as her mobility hasn't changed.  She only has a strong reaction when being pet in that spot.